September 9th 2016 by Matt An

September 5th
1. the use of bisects in python
binary search:
when we want to search an element in a list, we can take the middle value, find out the target is bigger than the middle or not, then we can abandon half of a list. keep on doing it, we can find it at last. the maximum time we take is log2n.
2. the example
2.1 iteration binary search
use iteration to make a binary search function, it's faster than recursion in binary search. but iteration use counter as the condition of a loop, when all the number count off, the loop will finish.

def search_binary_iter(xs, target):
    low=0
    high=len(xs)-1
    while low<=high:
        middle=(low+high)//2
        if xs[middle]<target:
            low=middle+1
        elif xs[middle]>target:
            high=middle-1
        else:
            return middle
    return -1

2.2 recursive binary search
recursion use the output as input again, the code will shorter than iteration, but it's slower,use more RAM.

def search_binary_recur(xs, target):
    low=0
    high=len(xs)-1
    middle=(low+high)//2
    if low==high:
        return -1
    else:
      if xs[middle]==target:
         return middle
      elif xs[middle]>target:
        return search_binary_recur(xs[:middle,target])
      elif xs[middle]<target:
         return search_binary_recur(xs[middle:,target])
      else:
        return -1